Hopper’s earliest form? Piece in American bar (7)
I believe the answer is:
tadpole
'hopper's earliest form?' is the definition.
(early form of a frog or toad)
'piece in american bar' is the wordplay.
'piece in american' becomes 'tad' (US term for a small amount).
'bar' becomes 'pole'.
'tad'+'pole'='TADPOLE'
(Other definitions for tadpole that I've seen before include "Young Kermit" , "Legless creature" , "Larval frog" , "Aquatic larva of toads and frogs" , "young swimmer" .)